In membrane technology, porous films are made from polymer solutions by spreading a thin layer of polymer solution on a suitable flat bottom layer (substrate), for example a glass plate. The bottom layer is then immersed together with the polymer solution in a nonsolvent for the polymer. The polymer solution will separate (precipitate), and this results in a particular porous structure in the polymeric material which has then solidified. This process is termed coagulation. After coagulation, the membrane can be removed from the glass plate and used as is. This method is in principle much used for producing reverse osmosis and ultrafiltration membranes, in particular of nonelastomeric polymers.
A specific membrane having a strongly asymmetrical structure and suitable for use as a wound covering material (artificial skin) is described in the Dutch Patent Application NL-A-8801741, which corresponds to a considerable extent to EP-A-0,351,016 (publication date Jan. 17, 1990). The wound covering material has a gradient in the pore size distribution viewed in the cross section of the material, and this implies that there is a top layer which is in contact with the environment and possesses pores of less than 0.5 .mu.m, while the side which is in contact with the damaged skin (bottom layer) possesses relatively large pores in the region of 20-200 .mu.m. The wound covering material is produced by a method in which the starting point is a polymer solution (preferably polyurethanes) to which particles (for example salt particles) have been added. These solid particles have a twofold purpose. Firstly, they serve to prevent shrinkage phenomena which occur during and shortly after the coagulation process; secondly they serve as so-called pore formers. The relatively large pores in the bottom layer are obtained by adding particles with the correct dimensions to the polymer solution. These particles which are washed out, after coagulation of the polymer solution, using a solvent suitable for said particles, therefore leave behind pores having a particular size and, in addition, prevent shrinkage phenomena in the precipitating polymer solution. It will be clear that the solvent used for washing out has to be an agent other than the coagulating medium for the polymer solution so that the particles are not already washed out in the coagulating medium. The polymer therefore has first to be precipitated and then the salt particles still present have to be washed out. Suitable particles are, for example, crystals of the salt sodium citrate. The thin top layer having the small pores present therein can be obtained from a similar solution (but now without salt crystals).
The method described is very time-consuming and is only suitable for obtaining small surfaces (a few cm.sup.2) of the wound covering described on a laboratory scale. The very thin top layer (thickness 0.01-0.2 mm) is very difficult to apply, while the use of two different nonsolvents also presents very many production problems.
The use of elastomeric materials for producing imitation leather is also known from the literature. An important property of imitation leather is the permeability to water vapour, the material nevertheless having to have the necessary mechanical strength. Imitation leather can also be made by starting from a polymer solution and a coagulation process in which a product is eventually obtained which is composed of two layers: a thin layer having small pores is applied to a porous carrier and eventually forms an integral whole with said porous carrier. The pore size in the porous carrier layer is in general not larger than a few micrometers, while the pore size in the top layer is not described. It will be clear that, in view of the applications of imitation leather, the microporous top layer has to have a very good and lasting adherence to the porous substrate layer.
The problem underlying the present invention was to provide a method for producing a flat porous product which is suitable, for example, as wound covering material, which method can in principle be carried out in one step, the use of two different nonsolvents being avoided and the desired pore structure nevertheless being obtained.
This problem is solved according to the invention.